Henry Piaget answers your question, he proposed that there is a special phase in a childs mental development where the brain accords itself to the wovel positions, fundamental syntactic rules and a set of other properties specific to one´s own native language. Once this language acquisition window is closed it may be virtually impossible for an individual to truely acquire a language. Bilingualism or trilingualism is possible in practice but in reality no matter how many languages one can speak, only one of them is his/her native language. It is generally accepted that the language that you do the calculations with is your native one.

Unless, you are giving up your identity or thinking of working as a spy in another country, don´t ever think of becoming a native speaker.  There are a lot more to add on top of the mastery one gains in the early years of life. These include local dialects, thinking patterns, accents, jargons and slang...

ok, but if you believe what Vygotsky says about people making sort of an "inner speach", loudly as a playing child, and silent as an adult, then how Piaget would explain that what ever language inviroment people are in, they are THINKING in that language, too, building their sentences inside themselves in the language that they are surrounded with (although it´s far from being their native language)? 

That does not stray from the basic tenet of Piaget at all. Of course, language acquisition theories are too voluminous to comprehensively discuss here. According to Piaget, a baby learns how to use its tongue just as the way it learns how to use its other organs.  Many skills and talents are learned the difficult way, just like the way blacksmith makes a sword on anvil by pounding on it with his hammer for hours. Once formed, it becomes perfect. All native speakers are thought to have a perfect command of their languages unless they are impaired. You might argue, there must be a difference among people´s language skills according to vocabulary use and the level of eloquence. True but all the extra skills are indeed extras and they reside in the intellectual capacities of individuals rather than in their linguistic proficiency. A garbage collector can not speak like say the President but on the other hand the President will not be able to realisticly mimic a garbage collectors use of the language either.

There are situations where I can overwhelm say a New Jersey woman with my knowledge of vocabulary. She may not understand the meanings of a number of words I might say but on the whole my command of English is not even comparable to hers.  She will always feel more comfortable in English. People will always understand her with greater ease and she will perform her daily tasks a lot more easier than a non-native speaker. In the evening, I may draw a volume of Paradise Lost by Milton  from my book shelf and enjoy reading it. Many native speakers can not comprehensively read a book like that. This l however, will not change the fact that my English is poor by their standards.

 i´ll save it to my computer...maybe it will help me in january (exam)...now back to book, next chapter - Bloom´s Taxonomy... but thanks, you are telling those things much easyer than my book (about 750 pages ).

 what is the mistakes.?

 if ihave any mistakes that is because iamwritting fast and iam angry so ican write good

Do you know iam English teacher in my country!

 Do you really want to know? Ok, I´ll show it to you in your post above.

 And I didn´t mark your lack of using capitals. (BTW if there are things I overlooked or did wrong, I´ll be happy to be corrected by a real native speaker.)

 Merhaba saskia1970

iam writting fast and iam angry because we make that for me

what is wrong?

in my first question how ican be come perfect in turkish?

so are you test me in english to talk that you are like me not native

what is problem ?

SO Do you know iam english teacher in my country

that is the prove of my mistakes becomes because iam angry and nerves

 Saskia1970

that is too much  why you are do that?

My friend also is english teacher...but this doesnt prove anything about her english. All i can say about her english that is better than mine. Nothing else.